Amplitude modulated receivers are known in the art which operate in a signal seeking or search mode and include a stop circuit which generates a stop signal to deactivate the signal seeking mode. One prior art stopping circuit includes a first comparator, or level detector which compares the rectified and filtered IF singal with a threshold potential established by a biasing network to detect when the receiver is on-channel. The prior art circuit also includes a second comparator which compares the level of the detected audio signal to another threshold potential established by a second biasing network. The outputs of the two comparators are used to generate a stop signal when the receiver is on-channel and a desired signal strength is present. One of the problems associated with this configuration is that a low bias threshold level is required and other variables are introduced which in effect lower and compress the comparator operating thresholds to a marginal level. At this marginal level, temperature variations and offset voltage fluctuations can cause erroneous operation of the stopping circuit manifested by stopping prior to, or subsequent to, the center of the channel or skipping desired channels. Also, in receivers including a local/distant mode of operation, two different signal strengths must be detected. In the past, a compromise in the local and distant mode sensitivity setting has been necessary.
It is an object of the present invention to provide a signal seeking stop circuit for an amplitude modulated receiver which improves on-channel stopping characteristics without degrading the receiver's overall performance.
It is another object of the present invention is to provide an amplitude modulated receiver operative in a signal seeking mode wherein sensitivity adjustment for a local mode of operation is independent of the distant mode sensitivity setting.